In the process of electrophotographic or xerographic printing, a photoconductive member is employed to record an image. Illustratively, the photoconductive member, in the form of a belt or a drum, is charged to a substantially uniform potential to sensitize its photosensitive surface. In the case of a copying machine, a light source illuminates an original document to be copied Through the use of lenses, mirrors, and various other optical components, the charged portion of the photoconductive surface is exposed to a reflected light image of the original document to be reproduced. The photoconductive surface exposed to the light becomes conducting and its potential is reduced; the unexposed surface, i.e., the line - or print - covered part of the image, remains non-conducting and its potential is unchanged In this way the light image is recorded as an electrostatic latent image on the photoconductive member.
In the case of an electrographic printer connected to a computer, a similar process is used to record information on the photoconductive member The charged portion of the photoconductive surface is exposed to a light image produced by an optical print head. The precise shape of the light image is controlled by input signals from the computer For example, a laser or an LED array may be used as an optical print head which receives input signals from the computer to illuminate the photoconductive member with a light image of a particular shape. Here too, an electrostatic latent image corresponding to the desired information areas is recorded on the photoconductive member as areas of higher and lower potential.
A variation of the foregoing process utilizes a photoconductive member covered with a thin, transparent insulating layer; this is the basis for the Katsuragawa electrophotographic process which is described below.
In the Katsuragawa process, the basic photoconductive member is a three layer sandwich comprising a conductive substrate, a photoconductive layer, and a thin transparent dielectric layer covering the photoconductive layer. The steps involved in forming the electrostatic image by this process are: (1) corona charging to produce a surface potential of one polarity on the photoconductive member, (2) reverse polarity corona charging simultaneous with image exposure, and (3) overall uniform illumination. As a result of the above steps, the final latent electrostatic image resides on the surface of the dielectric layer and the field within the photoconductive layer is reduced to zero. From this point on, the operational steps are the same as in xerography: development of the latent electrostatic image with toner, transfer of the toner image to paper, fixing the toner on the paper and cleaning of the drum to prepare for the next cycle.
After recording the electrostatic latent image on the photoconductive member, whether by standard xerographic techniques or by the Katsuragawa process, the latent image is developed by bringing charged developer material or toner into contact with it. The charged developer material is attracted to the information areas of the electrostatic latent image and forms a developed or powder image on the photoconductive member corresponding to the electrostatic latent image. The powder image is subsequently transferred to a sheet of recording medium, such as a sheet of paper, in a transfer region. Thereafter, the powder image is permanently affixed to this sheet by a variety of methods, the most common of which is by fusing.
As the above-described processes utilize only one toner station, the image is printed in one color only, that is, the color of the toner which, most commonly, is black. It is evident that in order to achieve a multi-color print, a variety of toners must be used, each having a different color and each forming a corresponding portion of the image.
Several approaches to the implementation of multi-color electrographic printing can be found in the prior art. U.S. Pat. No. 2,584,695 to P.J. Good teaches a method of successive scanning of an original document under different color light filters, and successive printing on the print medium in each different color. Each color requires an individual scan so this process is a repetition of the single color process for each color.
U.S. Pat. No. 2,752,833 to C.W. Jacob also teaches a process for multi-color electrographic printing which is a repetition of the single color process for each color. In this process, a three gun cathode ray tube generates three images of an object scanned through three colored filters. The image patterns formed on the face of the CRT are directed via lenses and mirrors to three different areas of the interior of a photoconductive drum. A print medium is wrapped around the outside of the drum. Three successive images are formed on the drum, as it rotates. Following formation of each image there is a development and print cycle in each of the three different colors. This method is similar to the previous method insofar as it depends on the sequential repetition of the single color process.
A different approach is taught in U.S. Pat. No. 2,962,374 to J.H. Dessauer. This method employs a multi-layer photoconductive medium, where the first layer responds to a first color and transmits the remaining colors; the next layer responds to a second color and transmits the remaining color; and the last layer responds to the last color. The three photoconductive layers are developed separately and the images are superimposed in the printing steps which occur sequentially.
All of the aforementioned methods of electrophotographic printing are subject to the problems of resolution and registration inherent in the process of forming, developing and superimposing several different color component images in order to create one multi-colored composite image.
U.S. Pat. No. 2,879,397 to E.H. Lehmann relates to a dual development procedure of electroradiographic latent images. These latent images are formed when x-ray patterns are projected on a recording device made of materials whose electrical conductivity is altered by exposure to penetrating radiation such as x-rays and gamma-rays. The patent teaches successive development steps, in the first of which development is carried out in a gas suspension of particles of one color and electrical polarity, followed by a second development step in a gas suspension of particles of contrasting color and opposite polarity. This results in a two color image with enhanced detail in all developed image areas.
Japanese Patent Disclosures 54-7337 and 59-102257 both disclose copying machines which employ the Katsuragawa process.
Japanese Patent Disclosure Document 59-102257 teaches a 2-color copying apparatus which employs the Katsuragawa process. The image to be copied may be viewed as comprising black and blue regions, red regions, and yellow and white regions. The copier disclosed in the aforementioned Japanese patent document employs a photoconductive member comprising a conductive substrate, a photoconductive layer on top of the conductive substrate, and a thin transparent insulating layer on top of the photoconductive layer. The photoconductive member is insensitive to blackness (i.e., the absence of light) and to blue light, but is sensitive to red, yellow, and white light. The 2-color copying process disclosed in the Japanese patent document works in the following manner. Initially, a first electric charging device applies a uniform electrostatic charge of a given polarity to the surface of the photoconductive member. Next, a second electric charging device of opposite polarity is applied to the photoconductive member while simultaneously a first reflected light image lacking a certain color (e.g., red) due to filtering is projected onto the photoconductive member to form a first latent electrostatic image. This first latent electrostatic image corresponds to the yellow and white regions of the image to be copied since the photoconductive member is insensitive to black and blue, while red has been filtered out. Next, the photoconductive drum is irradiated with a reflected light image of the color (e.g., red) which was removed from the first light image to form a second latent electrostatic image. This second latent electrostatic image corresponds to the red regions of the image to be copied. Thus, there are now three distinct types of regions on the photoconductive member. A first type of region corresponds to the blue and black regions of the image to be copied, a second type of region corresponds to the white and yellow regions of the image to be copied and forms a first latent electrostatic image, and a third type of region corresponds to the red regions of the image to be copied and forms a second latent electrostatic image.
Next, the second latent electrostatic image corresponding to red regions is developed with a red toner. Thereafter, the photoconductive member is subjected to an overall charging process followed by uniform illumination. Subsequently, the regions on the photoconductive member corresponding to the black and blue regions of the image to be copied are developed, e.g., with a black toner, while the first latent electrostatic image corresponding to the yellow and white regions of the image to be copied remains undeveloped. Finally, the two separate developed images (blue-black and red) are transferred to a recording medium and fused thereto. The first (yellow-white) latent image, not having been developed, appears as white on the recording medium.
It should be noted that the toner used in both developing steps has the same (e.g., negative) electrostatic charge. Further, the second developing step (i.e., the development of the non-exposed blue-black regions) is a non-contact step so as not to disturb the previously developed second (red) latent image. It is a significant advantage of the copying process disclosed in the Japanese patent document that the entire two-color copying process can be accomplished during a single rotation of the photoconductive member.
The method and apparatus disclosed in Japanese Patent Disclosure Document 59-102257 are directed specifically to a twocolor copier. As such, the method and apparatus disclosed therein rely upon special devices or techniques for filtering out light of particular colors from an image to be copied in both the first and second irradiations. The method and apparatus disclosed therein are not easily adapted to a xerographic printer which is connected as an output device to a computer or other digital processing unit.
There is nothing in the prior art which shows how a xerographic printer which connects to a computer can be adapted to form a plurality of latent images on a photoconductive member, these latent images being developed into a multi-colored toned image in a single rotation of the photoconductive member.
It is therefore an object of this invention to provide a xerographic printer connected as an output device for a computer or other digital processing unit, which printer produces a plurality of latent images that are developed into a multi-color toned image in a single rotation or "pass" of a photoconductive member in the form of a drum or belt.
It is a further object of this invention to provide an electrophotographic printer which connects to a computer and which utilizes the Katsuragawa process to form a multi-colored toned image in a single rotation of an electrophotoconductive member such as a drum or belt.